Optical signal transmission and transportation is a key enabler and driving force in today's high speed digital communication infrastructure that supports vast amount of data transportation essential for many data centric informational applications such as, for example, internet application. With ever increasing demand for transportation bandwidth, new systems of optical signal transmission and transportation are constantly being developed that trend toward providing higher digital data rate and higher channel density count.
A digital optical signal, in a format of binary or multi-level, usually experiences certain amount of distortion during transportation that, together with other factors such as noise, affects overall system performance. Generally, the higher the data rate and the longer the distance travelled by the optical signal, the bigger amount of distortion that the optical signal experiences. Among many factors contributing to the optical signal distortion, chromatic dispersion of the transportation media such as optical fiber is a major contributor. The amount of dispersion that an optical signal may be able to tolerate in a transportation system varies inversely proportional to the square of the data rate. As a general rule of thumb, for a 40 Gb/s direct detection system, the dispersion window, within which system performance variation is tolerable, is typically less than the equivalent of a piece of 10 km SMF-28 fiber measured at 1550 nm wavelength.
FIG. 1 is a simplified functional block diagram of an optical signal transportation system with line protection scheme as is known in the art. Under normal working conditions, optical signals are transported over working fiber pair such as paths 31 and 32, as a bidirectional optical transportation system 1, between terminal 10 and terminal 20. When there is a fault such as fiber cut in one or both of the working fiber paths 31 and 32, the amount of optical signal received at photo-detector PD2 in terminal 10 and/or at photo-detector PD5 in terminal 20 will generally decrease to a level below a pre-defined threshold. As a result, this decrease in received signal level triggers optical line protection (OLP) switches, such as SW1 and SW2 in terminal 10 and SW3 and SW4 in terminal 20, to switch and cause the system to transmit and receive optical signals via protection fiber pair such as paths 41 and 42, replacing working fiber paths 31 and 32. The same event, such as fiber cut, may also trigger the generation of a system alarm to alert the happening and existence of such fault in the working fiber paths 31 and 32. Optical signals transported over protection fiber paths 41 and 42 may continue to provide data service and be monitored by photo-detectors PD3 and PD6 while working fiber paths 31 and 32 are being repaired or restored. In the bidirectional optical transportation system 1 illustrated in FIG. 1, photo-detectors PD1 and PD4 are used to monitor optical signal levels launched into the fibers, in both directions.
However, configuration of the above optical system may not work well, where the system has a narrow dispersion tolerance window, due to difference in total fiber link dispersion between the working fiber path 31 or 32 and the protection fiber path 41 or 42, respectively. This is especially true in a DQPSK system where data rate of the optical signal is generally high, around 40 Gb/s or higher. Generally in the above system, in order to expand dispersion window that the optical signal and system are able to withstand, fiber-Bragg gratings (FBG) and more frequently Etalon-based channelized tunable dispersion compensation modules (TDCM), both of which are not shown in FIG. 1, are used at the receiving end of each channels of their respective terminals to bring down the total net dispersion.
In order to get the optical transportation system back to work or recovered, the tunable dispersion compensation module (TDCM) in each receiving channel is required to change or modify their dispersion setting so as to compensate any difference in the amount of dispersion between the working and the protection fiber paths. However, dispersion of this narrow band channelized TDCM is normally tuned through gradual temperature change which is usually in the range of seconds, if not in the tens of seconds. Together with the process of using forward error correction (FEC) feedback or other feedback mechanism to find the right setting for the TDCM, the entire process of recovering the transportation system from fiber cut for just one channel, for example, may take several seconds and sometimes close to tens of seconds and is thus considered slow from a system standpoint. It is known in the industry that for dynamic line protection application, it is generally required that the system recovery time be less than 50 ms. Obviously, the above thermally-tuned TDCM is unable to meet the 50 ms system recovery time requirements for the protection scheme of an optical transmission and transportation system.